It is known that volatile products such as perfluoromethanesulphonyl fluoride or perfluroethanesulphonyl fluoride may be recovered from the electrolyte used in ECF together with formed hydrogen by distillation. The product has thereby to be separated from hydrogen, which is a troublesome process step when working in industrial scale. A further disadvantage of this method is loss of HF from the electrolyte due to evaporation from the electrochemical cell, even if the cell is provided with a reflux condenser.
U.S. Pat. No. 5,322,597 discloses a process for separating perfluorinated products from the electrolyte in ECF by extraction. In the case of volatile products (e.g. perfluoro-methanesulphonyl fluoride) experiments have shown that extraction of the electrolyte is not sufficient in order to separate hydrogen and product effectively. A considerably amount of the product is flushed out of the process mixture with hydrogen gas because of the high vapour pressure of the product.
It is further known to precipitate the product directly from the electrolyte, if the product has a low solubility in HF at the employed temperatures (F. G. Drakesmith, D. A. Hughes, J. Appl. Electrochem., 9 (1979), p. 685; DE Patent No. 4,218,562 and JP Patent Application No. 92,232,289).
Under usual electrolysis conditions, it is only possible to precipitate perfluoro-n-alkanesulphonyl fluorides with more than 4 carbon atoms. Lower alkanesulphonyl fluorides can only be separated by excessive cooling of the product solution, which demands very low temperatures and complicates the process. Precipitation results furthermore in saturation of the electrolyte with perfluorinated product. Hollitzer and Sartori (J. Fluorine Chem., 35 (1987), 329-341) have shown that during synthesis of perfluropropanesulphonyl fluoride high product yields cannot be obtained, when the product is accumulated in the cell after formation due to decomposition reactions of the product. Gramstad and Haszeldine (J. Chem.- Soc., 1957, p. 2640) have additionally observed decomposition of C--C and C-S-bindings during ECF of alkanesulphonyl fluorides.
It is thus necessary to keep the concentration of dissolved products by the electrolyte as low as possible, which is not possible by the above precipitation method.